ASPECT DEPENDENCE 



425 



BEAM 



STERN 



Curve 



Test 



San Diego measurements on moving vessels. 



— — — New York measurements on anchored vessels. 

 New York measurements on moving vessels. 



Figure 1. Aspect dependence for all tests. 



than values at any other aspect, almost 20 db higher 

 than the next highest value, and more than 40 db 

 higher than the average target strength at off-beam 

 aspects. Such a peak would be expected theoreti- 

 cally from specular reflection from the broadside of 

 the ship at beam aspect ; at a few degrees away from 

 beam aspect, however, the target strength should be 

 markedly reduced. 



This peak in Figure 2 may be exaggerated for two 

 reasons, so that the actual aspect dependence may 

 not be so sharp as it appears. First, the two observa- 

 tions constituting the peak were made at ranges be- 

 tween 500 and 600 yd, but most of the other observa- 

 tions were made at much closer ranges. Since the sur- 



70 

 60 



<n 50 



-I 



bj 



a 40 



a 



z 



X 30 



z 



UJ 



c 20 



Wl 



g 10 



g 







I ■"■ — -^— ' ■■■■ 



* • 



-. ^^r— 



__! t 1- 



Figure 2. Aspect dependence for still vessels (New 

 York). 



face vessel target strengths were also foimd to de- 

 pend on the range, increasing as the range increased, 

 especially at beam aspect, these high values may be 

 more the result of the effect of the range on the tar- 

 get strength, than the effect of the aspect angle of 

 the target. The data were too few to permit separa- 

 tion of these two factors and independent evaluation 

 of the effect of each on the measured target strengths. 

 Secondly, so few observations are plotted in Fig- 

 ure 2 that two very marked peaks may not be re- 

 liable. Only 31 values were obtained in the New 

 York tests on anchored vessels, and only 8 of these 

 were at aspects within 20 degrees of the beam. In 

 view of the large scatter of values, the observations 

 cannot be considered conclusive, but are at least gen- 

 erally consistent with the theoretical expectation 

 that still vessels reflect sound specularly. 



24.3.2 



Moving Vessels 



The dependence of target strength on aspect angle 

 for moving vessels is much less than the dependence 

 found for still vessels. The small variation in Table 1 

 is too small to be very significant. For comparison 

 with Figure 2, the individual target strengths for 

 moving ships measured at New York are plotted in 

 Figure 3 and for the destroyers measured at San 

 Diego in Figure 4. The scatter is so large that any 

 possible systematic variation of target strength with 

 aspect angle is largely obscured. 



Analysis of the San Diego data on moving vessels 



